Field
The present disclosure generally relates to the design of a hybrid optical source. More specifically, the present disclosure relates to the design of a hybrid optical source with reduced temperature sensitivity.
Related Art
Optical interconnects or links based on silicon photonics have the potential to alleviate inter-chip communication bottlenecks in high-performance computing systems that include multiple processor chips and memory chips. This is because, relative to electrical interconnects, optical interconnects offer significantly improved: bandwidth, density, power consumption, latency, and range.
In order to make a very low power (for example, less than 1 pJ/bit) optical interconnect, a power-efficient optical source, such as a semiconductor laser or a laser source, that is compatible with silicon-on-insulator (SOI) platforms is highly desirable. However, silicon cannot efficiently emit light because of the fundamental limitations of its indirect bandgap and low mobility. Consequently, one approach for implementing silicon lasers is to integrate discrete III-V semiconductor optical amplifiers with silicon-based optical devices in a hybrid optical source. In these approaches, the III-V semiconductor provides the optical gain (and, thus, the initial light), and the silicon-based optical devices achieve lasing-wavelength control by providing a narrow-band optical filter.
For thick SOI platforms, etched grating-based distributed Bragg reflectors (DBR) are typically used as wavelength-selective optical devices. Because of the thick silicon core (and, thus, the large optical mode size), the DBR can usually be butt-coupled with commercial III-IV laser diodes without excessive optical-mode-mismatch loss. However, there are often problems associated with this configuration. In particular, in order to transport light out of the laser, the DBR typically is used as a partial reflector, which simultaneously controls the total mirror loss of the external laser and the output/transmission into the silicon optical waveguide. Moreover, because of transmission loss, the etched gratings usually are very shallow so that they represent small perturbations. Consequently, in order to obtain sufficient reflectance, the total length of the DBR is typically on the order of a hundred microns. With this total length of the DBR, the whole external optical cavity is usually longer than a few millimeters. Cavities this long usually result in a dense longitudinal mode spacing, which often results in frequent optical-mode hopping under un-cooled operating conditions. Furthermore, laser-wavelength control in this configuration can be very difficult. In particular, currently an efficient tuning mechanism for a DBR having a long total length is unavailable. Therefore, the hybrid optical source typically needs to be mounted on a costly, bulky (usually several millimeters in size) thermoelectric cooler in order to achieve stabilized lasing.
Alternatively, for a thin (usually sub-micron) SOI platform, silicon-based ring resonators can be used as narrow-band filters for lasing-wavelength control. Because the substrates in such SOI platforms are usually thin, this type of hybrid optical source is typically very compact and can be easily tuned, e.g., with built-in thermal heaters. However, silicon-based ring resonators have a short free spectrum range (FSR), so there are many repeated transmission/reflection maxima or minima in every FSR in the frequency domain. Moreover, in order to ensure single wavelength lasing, the 3 dB gain-spectrum range of a semiconductor laser diode is usually around 20-30 nm. Because the FSR of the ring reflector is typically designed to be larger than this gain-spectrum range, the silicon-based ring resonator typically has a radius below 5 um. These small features often increase the bending loss, and fabricating them often involves advanced lithography techniques that increase the cost of the hybrid optical source.
Hence, what is needed is an optical source without the problems described above.